This invention relates to the monitoring of corrosion occuring to metal cables. Metal cables located in a hostile corrosive environment can be protected from corrosion by a variety of defences such as plastic cladding, grease or galvanising. For example, power cables mounted above ground comprise two or three layers of aluminum strands helically laid over a central core of seven galvanised steel strands. The steel core can also be greased. When moisture penetrates to the core of a cable wherein the grease layer is poor, through say use, the galvanised layer on the steel provides some protection. Eventually, however, the steel is exposed and the aluminium becomes anodic and subject to galvanic corrosion. Generally the severest corrosion then occurs on the inner aluminium strands and is invisible from exterior observation; therefore, detection of the corrosion is very difficult. Only in advance stages of this type of corrosion does bulging of the cable occur allowing visible detection. Although non-destructive tests such as infra-red scanning of energised lines can detect corrosion, the detection is only at the later stages of corrosion. During the process of corrosion both the electrical and mechanical properties of the cable are degradated and ultimately failure of the cable can occur.